The present disclosure relates to a semiconductor structure, and particularly to a dynamic random access memory (DRAM) cell including a finFET access transistor and a method of manufacturing the same.
Trench capacitors are used in a variety of semiconductor chips for high areal capacitance and low device leakage. Typically, a trench capacitor provides a capacitance in the range from 4 fF (femto-Farad) to 120 fF. A trench capacitor may be employed as a charge storage unit in a dynamic random access memory (DRAM), which may be provided as a stand-alone semiconductor chip, or may be embedded in a system-on-chip (SoC) semiconductor chip. A trench capacitor may also be employed in a variety of circuit applications such as a charge pump or a capacitive analog component in a radio-frequency (RF) circuit.
When a DRAM is formed in a bulk substrate, a dielectric “collar” was incorporated into a top portion of each trench capacitor. The collar is typically a thick semiconductor oxide portion, which provides in a high enough threshold voltage to prevent inversion of the adjacent substrate material which separates the source region of the access field effect transistor from the buried plate of the trench capacitors when a logic “high” signal is stored in the trench capacitor. As the size of DRAM cells is scaled down, the length and/or width of the trench capacitor become too small to support a thick enough collar, while also leaving room to provide a conformal fill for the inner electrode of the trench capacitor with a conductive material.
While use of a semiconductor-on-oxide (SOI) substrate is an option for providing sufficient electrical isolation between access transistors and the buried plate (which is the outer electrode for the trench capacitors), SOI substrates are more expensive than bulk substrate. In addition, methods for forming a buried insulator layer within a bulk substrate generally require complex processing steps. Thus, a scheme is desired for circumventing the problems of a collar oxide and/or buried insulator formation for trench capacitors.